The overall aim of the proposed study is to determine the functional role of the calcium binding protein "calmodulin" in vertebrate smooth muscle. We aim to characterize the protein in terms of its physicochemical properties and its interactions. We are giving particular attention to the interaction of calmodulin with those enzyme systems controlling and regulating cyclic nucleotide levels and contractile events in smooth muscle. We are currently involved in purifying to homogeneity the calmodulin sensitive cyclic nucleotide phosphodiesterase of chicken gizzard, and during the coming year we intend to characterize this enzyme in terms of its physicochemical properties and its functional role with respect to both calmodulin and the calmodulin-independent phosphodiesterases in this tissue. We are also using calmodulin-Sepharose affinity chromatography to isolate other components which interact with calmodulin in order to determine how many and which functions calmodulin subserves in smooth muscle.